Lithography processing is used for forming circuit patterns that form a semiconductor device on a wafer. In lithography processing, a mask with a pattern to be transcribed on a wafer is used. The mask includes a light shielding layer of chrome (Cr) on a substantially transparent mask substrate or a mask layer of a halftone phase shift layer. A resist layer is deposited for patterning the mask layer and electron beam write equipment is used for electron beam exposure, after which developing is performed and a resist pattern is formed. This resist pattern is used as an etching mask to selectively etch the mask layer in order to form a mask pattern. In order to check if the mask pattern has been formed accurately and according to specifications, a mask defect inspection is performed.
A mask defect inspection involves obtaining an image based on light from a backlight that is transmitted through the mask. Due to increasing reductions of design rules for semiconductor devices, the sizes of defects (which affect yield) are also decreasing. However, existing mask defect inspecting apparatuses are insufficiently capable of detecting smaller defects.
Masks have pellicles attached in order to protect the mask patterns from external impurities. Thus, in the inspection of mask defects, when taking the pellicles into consideration, a working distance (that is the actual distance between an immersion lens unit of a defect detector and the surface of a mask) is set. Keeping in mind the mounting of the pellicle, the inspection gap is set at approximately 6.3 mm. Thus, because the inspection gap is set at a substantial width, it is difficult to realize an immersion lens with a high number of apertures (NA). Because a high NA is difficult to realize, the resolution of an obtained image is relatively low, limiting detection of defects.
Accordingly, there is a need to develop a defect inspecting method with higher resolution that can detect defects of a smaller size that accompany finer mask patterns.